1. Field of the invention
The present invention relates to a novel method of production of polyvinyl chloride resin for paste processing.
2. Description of the prior art
Paste processing of a polyvinyl chloride resin comprises the preparation of a liquid plastisol by mixing a polyvinyl chloride plastisol prepared specifically for the paste processing with compounding ingredients, such as plasticizers, stabilizers and, when necessary, pigments, fillers and the like, curing of the liquid plastisol by a suitable method, such as molding, coating, dipping and the like, and melting by heating followed by solidification of the plastisol to obtain a cured product.
The flow property of the plastisol naturally affects the workability of the paste processing to a great extent and much effort has actually been paid for improvement of the flow property.
Along with the flow property of the plastisol, the degree of dispersion of powder compounding components into the liquid compounding components affects the properties, particularly appearance and strength, of the molded product to a great extent.
A plastisol contains aggregated resin particles formed by the aggregation of a number of small particles of the resin. When the plastisol contains rough and large aggregates of the resin particles without being dispersed into small particles, the flow property of the plastisol is adversely affected and furthermore problems, such as clogging of the plastisol during transportation, streaking during coating, rough surfaces on the molded product, decrease in gloss of the cured product, decrease of strength in the cured product and like other problems, arise.
To overcome the problems of the paste processing described above, it is proposed that the material resin is supplied as a very fine powder which completely passes a sieve of 325 mesh of Tyler Standard Sieves. The fine powder is prepared by polymerizing vinyl chloride or a monomer mixture containing vinyl chloride as the main component in the presence of a radical generating polymerization initiator and an emulsifier by the method of emulsion polymerization or by the method of microsuspension polymerization. An aqueous suspension of spherical resin particles having a diameter of 0.05 to 5 .mu.m is obtained by the polymerization, which is then dried by spraying.
However, conventional resins prepared by the above method have problems caused by the fine powdery form of the resin, such as deteriorated working conditions because of scattering of the powder during charging into a bag, discharging from the bag for production of the plastisol and mixing in the production of the plastisol and difficulties in automatic weighing and automatic transportation because of the inferior flow property.
For overcoming the problems described above, a method of production of vinyl chloride resin for paste processing in which the polyvinyl chloride resin is recovered from a dispersion by separating it as aggregates from the aqueous phase with addition of an organic fluid which is at least barely soluble in water and does not dissolve or swell the polyvinyl chloride resin and then dried with or without granulation in advance is proposed in Japanese Patent Publication Heisei 1-42282.
Though the problems described above are improved, other problems, such as formation of blocking during the drying process, inhomogeneous drying and formation of particulate protrusions at the surface of the cured product by the presence of larger particles, are caused by the formation of too strong aggregates of the wet resin. The yield of recovery of the resin powder is about 96% at most and the 4% rest of the resin which is lost material creates a serious problem for a large production. It is absolutely necessary that the resin left in the aqueous phase is additionally effectively recovered.